Split-flow process and apparatus

ABSTRACT

A recovery plant for recovery of a gaseous component, such as carbon dioxide, from a process gas, such as the exhaust from a combustion turbine, has an absorber ( 110 ) employing a lean solvent ( 126 ) in an upper section of the absorber and a semi-lean solvent ( 172 ) in a lower section of the absorber. A regenerator ( 120 ) extracts the gaseous component from the rich solvent ( 117 ) thereby producing a regenerated lean solvent ( 126 ) and a semi-lean solvent ( 127 ). A solvent flow control element ( 170 ) combines at least a part of the semi-rich solvent ( 118 ) from the upper section of the absorber with the semi-lean solvent ( 127 ) from the regenerator to form a mixed solvent ( 171 ), and a cooler ( 112 ) cools the mixed solvent that is subsequently fed into the lower section of the absorber ( 110 ). By cooling the mixed solvent prior to entry into the absorber, the thermal energy required to remove the carbon dioxide from the exhaust gas can be reduced.

This application claims the benefit of U.S. provisional application No.60/109,613 filed Nov. 23, 1998, incorporated herein big reference in itsentirety.

FIELD OF THE INVENTION

The field of the invention is removal of a gaseous component from aprocess gas.

BACKGROUND OF THE INVENTION

Various methods are known in the art to remove a gaseous component froma stream of a process gas, including a wide range of distillation-,adsorption-, and absorption processes, and one relatively common processinvolves regenerator-absorber systems.

In a typical regenerator-absorber systems, gas is introduced in theabsorber where the gas contacts a lean solvent traveling down thecolumn. The gaseous component is at least partially absorbed by the leansolvent, and the purified process gas leaves the absorber for furtherprocessing or discharge. The lean solvent containing the gaseouscomponent (i.e. the rich solvent) flows through a cross heat exchangerthereby increasing its temperature. The heated rich solvent is thenstripped at low pressure in a regenerator. The stripped solvent (i.e.lean solvent) is sent back through the cross heat exchanger to reducethe temperature in the lean solvent before completing the loop back tothe absorber. The regenerator-absorber system process typically allowscontinuous operation of removal of a gaseous compound from a process gasat relatively low cost. However, the efficiency of removal of theGaseous component is not always satisfactory, and especially when thegaseous component is carbon dioxide, stringent emission standards canoften not be achieved with a standard regenerator-absorber system. Toovercome problems with low efficiency the temperature or pressure in theregenerator may be increased. However, corrosivity and solventdegradation generally limit the degree of optimization for this process.

An improved regenerator-absorber system is shown by Shoeld in U.S. Pat.No. 1,971,798 that comprises a split-loop absorption cycle, in which thebulk of the solvent is removed from an intermediate stage of theregenerator column and recycled to an intermediate stage of theabsorber. In this arrangement only a small portion of the solvent isstripped to the lowest concentration, and a high vapor to liquid ratiofor stripping is achieved in the bottom trays of the absorber, resultingin somewhat lower energy use at low outlet concentrations. However, thereduction in energy use is relatively low due to thermodynamicinefficiencies in stripping, mainly because of variations in the solventcomposition as it circulates within the split loop

To circumvent at least some of the problems with the split loop process,various improvements have been made. For example, one improvement to thesplit-loop process is to more accurately control the concentration ofsolvents. To more accurately control the solvent concentrations, twomodifications are generally necessary. The first modification comprisesan intermediate reboiler, which is installed to a main regenerator toboil off water from the semi-lean solvent to adjust the concentration ofthe semi-lean solvent stream to the concentration of the lean solvent.The second modification comprises a side-regenerator to regeneratecondensate from the main regenerator. The condensate from the mainregenerator is sent to the top section of the main regenerator, where itundergoes partial stripping, and is then further stripped to a very lowconcentration of dissolved gas in the side-regenerator, before beingreturned to the bottom reboiler of the main regenerator.

Since only a relatively small portion of the total solvent (typically˜20%) is stripped to the ultra-low concentration, the process allowsachieving relatively low outlet concentrations with comparably lowenergy use. Furthermore, when methyl diethanolamine (MDEA) is employedas a solvent in the improved split-loop process, the liquid circulationcan be reduced by about 20%. However, the modifications to improveenergy use and lower solvent circulation generally require a substantialmodification in the configuration of the main regenerator, and theinstallation of a side-regenerator, both of which may result insubstantial costs and significant down-time of an existingabsorber-regenerator system.

Another improvement to the split-loop process is described by Shethnaand Towler [“Gas Sweetening to Ultra-low Concentrations usingAlkanolamines Absorption”: Paper 46f, AlChE Spring Meeting, New Orleans1996], in which two regenerator columns are utilized. A primaryregenerator produces a semi-lean solvent, and a secondary regeneratorproduces an ultra-lean solvent. A small portion of the purified processgas leaving the absorber is expanded to a lower pressure level therebyproducing a cooled purified process gas. The heated ultra-lean solventstream leaving the secondary regenerator is cooled by the cooledpurified process gas thereby producing a heated purified process gas,which is subsequently fed into the secondary regenerator. The recycledgas is then recovered from the secondary regenerator and reintroducedinto the feed gas stream at the absorber.

The use of a substitute vapor instead of a reboiled solvent at thesecondary regenerator advantageously lowers the partial pressure of thesolvent vapor in the secondary regenerator, and allows the secondaryregenerator to operate a lower temperature than the primary regeneratorcolumn. Operating, the secondary regenerator at a reduced temperaturetypically results in a reduced corrosivity of the solvent, which in turnmay allow for the use of cheaper materials such as carbon steel in placeof the conventional stainless steel. Furthermore, a split-loop processusing vapor substitution may be combined with fixed-bed irreversibleabsorption technology, e.g. to remove H₂S and or COS from the recyclegas in a bed of solid sorbent, thereby ensuring a relatively long bedlife of the absorber. However, the split-loop process using vaporsubstitution requires the use of least two regenerator columns, and itmay further be necessary to re-tray the top stages of an existingabsorber to accommodate for the needs of this particular process.Moreover, due to the recycle gas and the use of a secondary regeneratorcolumn. retrofitting of existing absorber-regenerator combinations maybe relatively expensive and time consuming.

Although various improvements to the general layout of aabsorber-regenerator process have been known in the art, all or almostall of them suffer from one or more than one disadvantage. Therefore,there is a need to provide improved methods and apparatus for theremoval of a gaseous component from process gases.

SUMMARY OF THE INVENTION

The present invention is directed to a recovery plant to recover agaseous component from a process gas, having an absorber that employs alean solvent and a semi-lean solvent which absorb the gaseous componentfrom the process gas, thereby producing a rich solvent, a semi-richsolvent, and a lean process gas. A regenerator is coupled to theabsorber, wherein the regenerator extracts the gaseous component fromthe rich solvent, thereby regenerating the lean solvent and thesemi-lean solvent. A solvent flow control element is coupled to theabsorber and combines at least part of the semi-rich solvent with atleast part of the semi-lean solvent to form a mixed solvent. A cooler iscoupled to the absorber that cools the mixed solvent, and the cooledmixed solvent is subsequently fed into the absorber via a connectingelement.

In one aspect of the inventive subject matter, the process gas is a fluegas from a combustion turbine, having a pressure of less than 20 psiawhen fed into the absorber, and herein the gaseous component is carbondioxide. The concentration of carbon dioxide is preferably greater than2 mole %, more preferably greater than 5 mole %, and most preferablygreater than 10 mole %.

In another aspect of the inventive subject matter, the solvent comprisesa chemical solvent, preferably selected from the group consisting ofmonoethanolamine, diethanolamine, diglycolamine, andmethyldiethanolamine. It is also preferred that appropriate solventshave a concave equilibrium curve.

In a further aspect of the inventive subject matter, a method ofremoving a gaseous component from a process gas has a first step inwhich a stream of lean solvent and a stream of semi-lean solvent isprovided. In a second step, the process gas is contacted with the streamof lean solvent and semi-lean solvent in an absorber to produce a streamof semi-rich solvent and a stream of rich solvent. In a further step, atleast part of the semi-rich solvent and at least part of the semi-leansolvent are combined to form a mixed solvent stream in a still furtherstep the mixed solvent stream is cooled and the cooled mixed solventstream is introduced into the absorber to absorb the gaseous component.

Various objects, features, aspects and advantages of the presentinvention will become more apparent from the following detaileddescription of preferred embodiments of the invention. along with theaccompanying drawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an improved CO₂ removal plant according to theinventive subject matter.

FIG. 2 is a flow diagram of an improved process for CO₂ removal from aflue gas according to the inventive subject matter.

DETAILED DESCRIPTION

As used herein, the term “absorber” refers to an apparatus in whichsignificant amounts of at least one gaseous compound are removed from amixture of gaseous compounds, and that contains, when in operation, asolvent that is substantially selective towards the compound. The term“substantially selective” means that the solvent absorbs the compound toa significant higher degree (i.e. >20%) than the other compounds presentin the mixture of gaseous compounds. The solvent with the highestconcentration of the absorbed gaseous compound leaving the absorber istermed “rich solvent”, while the solvent with the lowest concentrationof the absorbed gaseous compound leaving the regenerator is termed “leansolvent”. The term “semi-lean solvent” refers to the solvent leaving theregenerator, which has a higher concentration of the absorbed gaseouscompound than the lean solvent. The solvent that leaves the absorberhaving a lower concentration of the absorbed compound than the richsolvent is termed “semi-rich solvent”.

As also used herein, the term regenerator refers to an apparatus inwhich an absorbed gaseous compound is at least partially removed from arich solvent under elevated temperatures of about 110° C. to about 130°C.

In FIG. 1, a recover plant 100 has an absorber 110 and a regenerator120. A stream of process gas 160 comprising a gaseous component entersthe absorber, and a stream of rich solvent 117 leaves the absorber viarich solvent pump 111. The stream of rich solvent is heated in the crossheat exchanger 130 and enters the regenerator 120. The gaseous componentis carbon dioxide and is removed from the rich solvent in two differentlocations, whereby a semi-lean solvent stream 127 is generated at aposition higher that a lean solvent stream 126. Part of the lean solventstream 126 is redirected via the lean solvent pump 121 to the bottomreboiler 122, and enters again the regenerator 120. The gaseous compoundleaves the regenerator in a stream of gaseous compound 150, whilecarried over solvent is recovered and recycled from the stream ofgaseous compound via condenser 123 and condenser pump 125. Both the leansolvent stream and the semi-lean solvent stream are cooled in the crossheat exchanger 130, and the lean solvent stream 126 is further cooledvia additional cooler 114 before entering the absorber 110. Thesemi-lean solvent stream 127 is mixed in the solvent flow controlelement 170 with semi-rich solvent stream 118, which is delivered fromthe absorber 110 via semi-rich solvent stream pump 113. A mixed solventstream 171 is further cooled via cooler 112, and cooled mixed solventstream 172 enters the absorber via the connecting element 173 at a lowerposition than the lean solvent stream. A stream of lean process gas 140leaves the absorber 110 via the condenser 116, and a condenser pump 115pumps the condensate liquid back to the absorber.

In a preferred embodiment, the absorber 110 in carbon dioxide removalplant 100 is a random packed-bed absorber with a diameter of about 20 ftand a height of approximately 70 ft, configured to process about 30million standard cubic feet process gas per hour. The regenerator 120 isa standard regenerator with a diameter of about 10 ft and a height ofapproximately 50 ft, generating a stream of carbon dioxide 150. Processgas 160 is flue gas from a combustion turbine with a carbon dioxidecontent of about 13 mole % having a pressure of about 2 psig when ledinto the absorber, and the lean process gas 140 has a carbon dioxidecontent of about 3 mole %. The solvent in all solvent streams ismonoethanolamine, which absorbs the gaseous component carbon dioxide.The lean solvent stream 126 has a carbon dioxide loading of less than0.25, while the carbon dioxide loading in the semi-lean solvent stream127 is approximately 0.4. The rich solvent stream 117 has a carbondioxide loading of about 0.5, and the semi-rich solvent stream 118 has acarbon dioxide loading of greater than 0.3. The rich solvent stream 117is heated in a standard cross heat exchanger 130 before entering theregenerator, and both the lean solvent stream and the semi-lean solventstream are cooled in the cross heat exchanger. The solvent flow controlelement 170 is a static mixer with two inlet ports, and one outlet portthrough which the mixed solvent stream 171 exits the solvent flowcontrol element. Cooler 112 and 114 are standard side coolers with wateras a coolant or air coolers. Cooled mixed solvent stream 172 isapproximately 20° C. cooler than the mixed solvent stream 171, and isfed into the absorber via a line 173. The steam operated reboiler 122reheats a portion of the lean solvent stream 121 before recirculatingthe lean solvent stream into the regenerator. The pumps 111, 113, 115,121, and 125, condensers 116 and 123, and all lines are standardelements in plant for treatment of industrial gases, and well known tothe art.

In alternative aspects of the inventive subject matter, the absorberneed not be limited to a random packed-bed absorber with a diameter ofabout 20 ft and a height of approximately 70 ft, configured to processabout 30 million standard cubic feet per hour, but may include variousalternative types, sizes, and capacities. For example, where reducedcost of material is desirable, contemplated absorbers may includestructured packed-bed absorbers, While in applications that includecrude process gases, or gases with a relatively high degree ofimpurities, a trayed-type absorber may be employed. Similarly, whererelatively large capacities of process gas are to be purified, multipleabsorbers with same or different capacity may be utilized. Contemplatedprocess gas capacities include flow rates of between 1-50 millionstandard cubic feet per hour (MMSCF/hr), however larger flow ratesbetween 50-100 MMSCF/hr are also contemplated. Where smaller quantitiesof process gas are to be purified, flow rates of between 0.1-50 MMSCF/hrand less are contemplated. Consequently, the size of appropriateabsorbers may vary from 1 to 30 ft in diameter, and the height may varybetween 50 and 100 ft.

With respect to the process gas 160, it is contemplated that variousgases other than a flue gas from a combustion turbine with a carbondioxide content of about 13 mole % are also appropriate. For example,depending on the fuel source and combustion process, the carbon dioxidecontent may vary between less than 3 mole % and more than 20 mole %.Therefore, the carbon dioxide content may be greater than 2 mole %,greater than 5 mole %, and greater than 10 mole %. It should further beappreciated that gases other than flue gases from a combustion turbineare also contemplated, including natural gas, various refinery gases, orsteam reformer gases, all of which may or may not be pretreated.Contemplated pretreatment may thereby include fractionation, filtration,scrubbing, and combination or dilution with other gases. It is furthercontemplated that the pressure of the process gas need not be limited to2 psig when fed into the absorber, but may exhibit higher pressures.Contemplated higher pressures include pressures of less than about 20psia, less than 50 psia, less than 150 psia and less than 300 psia.

It is also contemplated that the solvent in all of the solvent streamsneed not be limited to monoethanolamine (MEA), but may comprise variousalternative solvents, including physical and chemical solvents, and anyreasonable combination thereof. For example, physical solvents includeSELEXOL™ (a dimethyl ether of polyethylene glycol) and methanol, whilechemical solvents include organic amines and mixed amines. Especiallycontemplated chemical solvents By are MEA, diethanolamine,diglycolamine, and methyldiethanolamine. It should further beappreciated that co-solvents in combination with contemplated solventare also appropriate. Furthermore, contemplated solvents may furthercomprise additives, including anti-oxidants, corrosion inhibitors, andanti-foam agents. With respect to the carbon dioxide loading of thevarious solvents it should be appreciated that the carbon dioxideloading may vary in the various solvents predominantly depending on thecarbon dioxide content of the process gas. Therefore, the data given forthe lean solvent stream, the semi-lean solvent stream, the semi-richsolvent stream, and the rich solvent stream are not intended to belimiting.

Furthermore, depending on the nature of the process gas and thephysico-chemical properties of the solvent, the stream of the gaseouscompound 150 is not limited to carbon dioxide, but may include hydrogensulfide, nitrogen, oxygen, hydrogen, helium, etc.

In further alternative aspects of the inventive subject matter, theregenerator may include various regenerators other than a standardregenerator with a diameter of about 10 ft, and a height ofapproximately 50 ft. For example, where relatively low amounts ofprocess gas are purified, smaller regenerators may be sufficient,whereas for the purification of relatively high amounts of process gas asingle larger regenerator or multiple regenerators are contemplated. Ingeneral, the regenerator is not limited in size or number so long asappropriate regenerators regenerate sufficient amounts of lean andsemi-lean solvent streams, and liberate the gaseous compound. Likewise,the reboiler 122 is not restricted to a steam operated reboiler, but mayalso be alternative reboilers, including oil-heated, or flame heated, orelectrically heated reboilers.

With respect to the heating of the rich solvent stream 117 and coolingof the lean solvent stream 126 and semi-lean solvent stream 127 it iscontemplated that various devices other than a cross heat exchanger arealso appropriate. For example, the rich solvent stream 117 may be heatedutilizing residual heat from the steam reboiler, or from heat sourcesother than a heat exchanger, including hot fluids, hot gases, andelectricity. It is especially contemplated that the heated rich solventstream is fed to the top of the regenerator in a single solvent stream,however, alternative configurations are also contemplated. Appropriateconfigurations include feeding the heated rich solvent at one or morethan one point at the side of the regenerator.

Similarly, the cooling of the lean solvent stream 126 and semi-leansolvent stream 127 may be performed with a single, or two independentcooling devices that employ water, air, or other refrigerants ascoolants. The cooling devices may thereby be energetically coupled orindependent from the gas purification process. Although the coolers 112and 114 are preferably side coolers coupled to the absorber, variousalterative configurations are also contemplated, including multiple sidecoolers or a single side cooler with two independent channels for thetwo solvent streams. In general, the size, nature of coolant, andcooling capacity are not limiting to the coolers, so long as the cooledmixed solvent stream is cooler than the mixed solvent stream, and solong as the cooled lean solvent stream is cooler than the lean solventstream. Contemplated coolers preferably reduce the temperature of thelean solvent stream and the mixed solvent stream more than 10° C., morepreferably more than 25° C., and most preferably more than 50° C. It isfurther contemplated that the connecting element 173 need not be limitedto a line, but may have various sizes, shapes, or forms so long as theconnecting element feeds the cooled mixed solvent into the absorber. Forexample, contemplated connecting elements include a simple opening, asingle, or multiple pipes or lines which may or may not be flexible, ora flange or other mounting means

In still further alterative aspects of the inventive subject matter, thesemi-rich solvent stream need not be limited to a single semi-richsolvent stream with a carbon dioxide loading of greater than 0.3, butmay include multiple semi-rich solvent streams with identical ordifferent carbon dioxide loading, so long as at least part of thesemi-rich solvent stream is mixed with at least part of the semi-leansolvent stream. For example, appropriate semi-rich solvent streams maybe drawn off the absorber at different positions that may or may nothave the same vertical distance from the top of the absorber.

With respect to the solvent flow control element 170 it is contemplatedthat various alternative devices other than a static mixer with twoinlet ports aid one outlet port are also appropriate, so long as atleast part of the semi-lean solvent stream is mixed with at least partof the semi-rich solvent stream. For example, one or more than onesimple T- or Y-shaped pipe connectors may already be sufficient,especially where portions of the cooler 112 may help in mixing the twosolvent streams. Where it is desirable to control the ratio of themixture of the two solvent streams, additional elements, including aflow control valve is contemplated. It should be especially appreciatedthat a configuration where a semi-lean solvent stream is mixed with asemi-rich solvent stream to form a mixed solvent stream, and where themixed solvent stream is cooled before entering an absorber reduces thethermal energy required to remove carbon dioxide from a flue gas.Further advantages of this design include an increase in solventcapacity, and a reduction of the solvent circulation rate.

In FIG. 2, a flow diagram 200 depicts a method of removing a Gaseouscomponent from a process gas, wherein in the first step 210 a stream oflean solvent and a stream of semi-lean solvent are provided. In a secondstep 220, the process gas is contacted in an absorber with the stream oflean solvent and the stream of semi-lean solvent to produce a stream ofsemi-rich solvent and a stream of rich solvent, and a subsequent step230 at least part of the stream of semi-rich solvent and at least partof the stream of semi-lean solvent are combined to form a mixed solventstream. In a further step 240 the mixed solvent stream is cooled and thecooled mixed solvent stream is introduced into the absorber to absorbthe gaseous component.

In a preferred embodiment the stream of lean solvent and the stream ofsemi-lean solvent both comprise MEA as a solvent, and are both producedby a regenerator. The process gas is a low-pressure flue gas from acombustion turbine with a pressure of less than 20 psia when fed intothe absorber, and the gaseous component in the flue gas is carbondioxide at a concentration of typically less than 20 mole %, and moretypically less than 10 mole %. The low pressure flue gas (i.e. less than100 psia when fed into the absorber) is contacted in an absorber with acounter current stream of lean solvent having a carbon dioxide loadingof about 0.2, and a stream of semi-lean solvent having a carbon dioxideloading of about 0.4, to produce a stream of semi-rich solvent having acarbon dioxide loading of more than 0.3 and a stream of rich solventhaving a carbon dioxide loading of about 0.5. Subsequently, the streamof semi-rich solvent and the stream of semi-lean solvent are combined ina static mixer to form a mixed solvent stream. Next, the mixed solventstream is cooled by a side cooler to form a cooled mixed solvent streamand the cooled mixed solvent stream is then fed into the absorber.

With respect to identical components between the preferred method ofFIG. 2 and preferred components of FIG. 1, the same considerations aspreviously discussed apply.

Thus, specific embodiments and applications of improved methods andapparatus for the removal of a gaseous component from a process gas havebeen disclosed. It should be apparent, however, to those skilled in theart that many more modifications besides those already described arepossible without departing from the inventive concepts herein. Theinventive subject matter, therefore, is not to be restricted except inthe spirit of the appended claims. Moreover, in interpreting both thespecification and the claims, all terms should be interpreted in thebroadest possible manner consistent with the context. In particular, theterms “comprises” and “comprising” should be interpreted as referring toelements, components, or steps in a non-exclusive manner, indicatingthat the referenced elements, components, or steps may be present, orutilized, or combined with other elements, components, or steps that arenot expressly referenced.

What is claimed is:
 1. A method of removing a gaseous component from aprocess gas, comprising: providing a lean solvent stream and a semi-leansolvent stream; feeding the process gas at a pressure between about 2psig to less than 20 psig in an absorber with the lean solvent streamand the semi-lean solvent stream to produce a semi-rich solvent streamand a rich solvent stream, wherein at least part of the lean solvent andat least part of the semi-lean solvent are produced by a regenerator;combining at least part of the semi-rich solvent stream and at leastpart of the semi-lean solvent stream to form a mixed solvent stream; andcooling the mixed solvent stream, and introducing the cooled mixedsolvent stream into the absorber to absorb the gaseous component.
 2. Themethod of claim 1 wherein the process gas comprises a flue gas.
 3. Themethod of claim 1 wherein the gaseous component is carbon dioxide. 4.The method of claim 1 wherein the carbon dioxide is present in theprocess gas at a concentration of less than 10 mole %.
 5. The method ofclaim 1 wherein the carbon dioxide is present in the process gas at aconcentration of less than 20 mole %.
 6. The method of claim 1 whereinthe lean solvent comprises monoethanolamine.